Double clicking a store by reference file in a container field causes FileMaker to use the path to the actual file in order to get the operating system to open that file with the default application associated with that file type. If there is no reference stored in the field, there is no separate file for the Operating System to open. In those cases, you must first export a copy of the file in order to produce a copy that can be opened. The export field contents script step can do this and includes an option for automatically opening the file.
As to why by reference pictures cannot be opened by double clicking, I can only speculate that the extra text indicating image size and that is inserted before the file reference prevents this from happening.
You can see what text is stored in a container field if you define a calculation field set to return text and put the container field in the specify calcualtion dialog for this field as the sole term. you can then place this field, sized several rows of text tall next to the container field it references to see the text change depending on what you insert in this field and how you insert it.
That's exactly what's happening with a picture, even when choosing only the file reference ...
In the calculated field you suggested, the first text is "size: 372,756 image:" .
After that the path and filename appear.
Is there any easy remedy?
Export Field Contents can open the image file for both all inserted files regardless of the insert options used. You can specify a file path to a location such as your temporary items folder so that you do not clutter your hard drive up with these extra copies.